Titanium oxide rectifier and method for manufacturing same



Feb. 11, 1958 KQJ] og 2,822,606

TITANIUM OXIDE RECTIFIER AND METHOD FOR MANUFACTURING SAME Filed Oct. 1, 1956 FiEJ.

forward current density /cm backward voltage (volts) I20 I00 80 60 40 20 I 2 s 4 5 forward voltage (volts) backward current density /cm) United States Patent:

TITANIUM OXIDE RECTIFIER AND METHOD F OR MANUFACTURING SAME Koji Yoshida, Kobe, Japan Application October 1, 1956, Serial No. 613,073

Claims priority, application Japan October 9, 1955 4 Claims. (Cl. 2925.3)

This invention relates to a titanium oxide rectifier and a method for manufacturing same.

An object of this invention is to provide a titanium oxide rectifier which is less costly and has characteristics and performances much more excellent than any known or conventional ones.

Another object is to provide a method for manufacturing the titanium oxide rectifier of such excellent characteristics. Other objects, features and advantages of this invention will become apparent from the following description and the accompanying drawings.

In the drawings:

Fig. l is a sectional view of a rectifier embodying this invention.

Fig. 2 is a graph illustrating the static characteristics of some of the rectifiers meoodying this invention as compared with the titanium oxide rectifiers to which this invention is not applied; wherein the vetrical coordinate is the amount of current and the horizontal coordinate denotes the voltage.

It has been intended to make a titanium oxide rectifier by heating a metallic titanium plate in an oxygen cont-aining atmosphere such as steam to form on the metal an imperfectly oxidized layer which acts as the semi-conductor, and then by fixing or closely sticking a metal such as lead, tin or gold upon the surface of the said layer by means of pressing, metal spraying, vacuum evaporation, plating or the like, so that the said metal acts as the counter electrode, while the metallic titanium plate constitutes another electrode. However, the rectifier of this structure is not satisfactory, particularly because of the fact that its back puncture voltage or backward voltage is not sufficient. In Fig. 2 the curve I shows the static characteristics of the rectifier of this type, which is made first by heating a metallic titanium plate in steam at 800 C. for 2 hours and at 900 C. for 20 minutes to form a layer of the titanium oxide semi-conductor, and then by pressfixing thereon lead as the counter electrode. It is clear from this curve that the rectifier made in this way is not satisfactory in its charatceristics, particularly in the backward voltage.

However, it has been found that, if the surface of the titanium oxide semi-conductor should further be oxidized to form thereupon a thin film or layer oxidized to a greater extent, the back puncture voltage or backward voltage is increased. For the purpose of this further or additional oxidation, anodic oxidation in an aqueous solu: tion has been attempted. For example, the imperfectly oxidized metallic titanium plate made by the above mentioned process is immersed in a bath containing aqueous solution of ammonium phosphate and is subjected to anodic oxidation for about 30 minutes with the bath voltage of about 200 volts. Upon the surface of the titanium oxide plate thus obtained, there is fixed a film of gold as the counter electrode. The curve II of Fig. 2 shows the static characteristics of this rectifier. Evident- 2 ly the rectifier of this type is better in its characteristics than the one where the oxidation is made only by heating in steam.

In such anodic oxidation, electric current will concentrate itself in the spots on the surface of the titanium oxide semiconductor where the resistance is low, so that these spots will be oxidized first and the resistance will be increased. As a result, there can uniformly be formed on that surface an oxidized thin film of a high resistance.

The present invention is based upon my discovery that, if fused salt or salts should be used in place of the aqueous solution in the anodic oxidation, such an uniformly oxidized thin and smooth film can more easily and effectively be formed than otherwise so that a rectifier of a far greater performance is obtained.

According to the observation through the photo electron micrograph, the surface of the oxidized film made in accordance with this invention is consider-ably smoother than that of the oxidized film made by the anodic oxidation in an aqueous solution. In other words, when an aqueous solution is used, the surface is sharply wrinkled, whereas such sharp wrinkles disappear and there are formed rounded rumples when a fused salt is used. Particularly when a fused alkaline salt or a mixture of alkaline salts is used together with other fused salt, such small rumples disappear and a very smooth surface is obtained. It is deemed that this is the reason why a rectifier showing high back puncture voltage or backward voltage is obtained according to this invention.

Referring to Fig. 1, there is shown a sectional view of a rectifier embodying this invention. Indicated with the reference numeral 1 is a metallic titanium plate or layer upon which a titanium oxide semi-conductor layer 2 is formed by heating the said titanium plate, for example, in steam. On the said layer 2, there is formed an oxidized thin film 3 by an enhanced anodic oxidation in fused salt(s) according to this invention. Indicated by the numeral 4 is 'a counter electrode fixed on the surface of the thin film 3. The counter electrode may be any known one.

As mentioned above, the method for the manufacture of the rectifiers according to this invention comprises the following steps:

(1) Imperfectly oxidizing a metallic titanium to form thereon a titanium oxide semi-conductor layer.

(2) Etfecting further or additional oxidation thereof to a greater extent by anodic oxidation in fused salt(s) to form another thin film or layer on the surface of the said semi-conductor.

(3) Fixing a counter electrode on the surface of said last formed film or layer.

The first or imperfect oxidation may he made by any known method. But, thermal oxidation especially by the use of steam is preferably because of the simplicity of its operation. The characteristics of a rectifier may be varied, although not greatly, by varying the conditions of such thermal oxidation. It has been found that when the thermal oxidation is made at a higher temperature the backward characteristics are further improved, while the for ward voltage increases consequently. Contrary, if the thermal oxidation is made at a lower temperature the reverse elfect is obtained.

The important feature of this invention lies in the sub sequent anodic oxidation in fused salt(s) having comparatively low melting points. As examples of salts which may be used for this invention, the following may be mentioned: sodium nitrate, potassium nitrate, sodium hydroxide, potassium hydroxide and a mixture of two or more or them. It has been found that when an alkaline salt or a mixture of alkaline salts such as sodium hydroxide and potassium hydroxide is used together with other salts(s), the surface of the further or additionally oxidized film or layer becomes smoother and the backward characteristics of the rectifier obtained thereby are improved.

Generally, the anodic oxidation in fused salt(s) according to this invention is carried'out as follows:

In an electrolytic bath containing a fused salt or salts as mentioned above, there are immersed a metallic titanium plate having thereon a semi-conductor layer and a metallic electrode made of a metal such as platinum or titanium which offers a good resistance to corrosion by such salts. The surface of the semiconductor layer and the metallic electrode constitute the anode and cathode respectively. To this bath the electric current is passed with a current density of 50 to 100 ma. per sq. cm. As the oxidation on the anode surface proceeds, the current density decreases. The treatment is completed within a few minutes, when the current density is reduced to a few ma. In this way, a thin film or layer which is oxidized to a greater extent is formed on the surface of the titanium semi-conductor layer.

On this oxidized film or layer, there is fixed a counter electrode which may be any known one. However, since one of the important features and advantages of titanium rectifiers lies in its resistance to high temperatures, the counter electrode made of a metal of a high melting point such as 300 C. or more is preferable. For example, gold, lead or tin is satisfactory. The counter electrode may be fixed on the said surface by any such suitable means as pressing, metal spraying, vacuum evaporation, plating or baking. However, when the counter electrode layer is fixed upon said surface by means of vacuum evaporation or baking the said layer is very thin so that its thermal conductivity is low and accordingly there occurs local overheating which causes deterioration of the counter electrode. As the surface of the oxidized film is smooth and hard it is difiicult to fix the counter electrode layer firmly on the said surface by means of metal spraying. It is preferable, although not necessary, to form on the said surface the counter electrode layer by means of vacuum evaporation or baking and then to effect metal spray thereon.

Some examples of this invention are given below. However, it should be noted that this invention is not limited to these particular examples.

Example 1 A metallic titanium plate is heated in steam at 800 C. for two hours and then at 900 C. for minutes to produce an imperfectly oxidized layer or semi-conductor on the surface of the said plate.

In an electrolytic bath containing a mixture of fused sodium nitrate and potassium hydroxide in the ratio of 2:1 by weight are immersed the imperfectly oxidized titanium plate as the anode and a metallic titanium plate as the cathode. Electric current is then passed thereto with the current density of 50 ma./cm. This treatment is completed within about five minutes, when the bath voltage attains its maximum value, namely, 60 v. and the current density is reduced to 2 ma./cm. In this way, a thin film or layer oxidized to a greater extent is formed on the surface of the semi-conductor layer.

As a counter electrode, a film of gold is fixed on the surface of the oxidized film or layer by baking liquid bright gold applied on it.

The static characteristics of the rectifier made in this way are shown in the curve III of Fig. 2. i

Example 2 A metallic titanium plate is heated in steam at 800 C. for two hours and then at 900 C. for an hour and at last at 1,000 C. for 10 minutes. 7

The subsequent treatment is same as Example 1. The static characteristics of the rectifier thus obtained are shown in the curve IV of Fig. 2, from which it is clear that its characteristics are approximately equal to those of the conventional three selenium rectifiers connected in series.

Example 3 A metallic titanium plate is subjected to the thermal oxidation in the same manner as Example 1.

In an electrolytic bath containing fused sodium nitrate are immersed the already or imperfectly oxidized titanium plate as the anode and a metallic titanium plate as the cathode. Anodic oxidation is made under the same conditions as Example 1.

Then, as the counter electrode, a film of gold is fixed on the surface of the further or additionally oxidized layer or film in the same manner as Example 1.

The static characteristics of the rectifier thus formed are shown in the curve V of Pig. 2.

The heat characteristics of the rectifiers of this invention are much more excellent than any known kinds of rectifier. For example, it should be noted that the reverse resistance of the rectifier according to this invention will attain its maximum in a temperature of about 200 C. and no deterioration is noticed until up to about 300 C. Because of the fact that the rectifier of this invention can be used at such a high temperature, it is possible to increase the current density, and increase in the back puncture voltage as well as in the current density makes it possible to greatly reduce the dimension of the rectifier.

According to this invention, as described above, a highly oxidized thin and uniform film or layer can be formed on the surface of a titanium oxide semi-conductor, so that reverse resistance is greatly increased and, consequently, the back puncture voltage or backward voltage is likewise increased, whereas forward resistance does not increase so much. As a result, this invention makes it feasible to manufacture the rectifier of such improved characteristics with a large rectification ratio. In addition, the rectifier of this invention is useful at a high temperature without any deterioration.

I claim:

1. A method for manufacturing a titanium oxide rectifier which comprises forming a layer of titanium oxide semi-conductor on a metallic titanium plate, oxidizing the surface of the titanium oxide semiconductor in fused salt or salts to form a thin film or layer oxidized to a greater extent, and fixing a counter electrode on said last References Cited in the file of this patent UNITED STATES PATENTS 2,749,595 Breckenridge June 12, 1956 

1. A METHOD FOR MANUFACTURING A TITANIUM OXIDE RECTIFIER WHICH COMPRISES FORMING A LAYER OF TITANIUM OXIDE SEMI-CONDUCTOR ON A METALLIC TITANIUM PLATE, OXIDIZING THE SURFACE OF THE TITANIUM OXIDE SEMI-CONDUCTOR IN FUSED SALT OR SALTS TO FORM A THIN FILM OR LAYER OXIDIZED TO A GREATER EXTENT, AND FIXING A COUNTER ELECTRODE ON SAID LAST MENTIONED FILM OR LAYER. 